Zhi-Hao Chen, Zhuang Liu,2,*, Lei Zhang, Quan-Wei Cai, Jia-Qi Hu, Wei Wang,2, Xiao-Jie Ju,2,Rui Xie,2, Liang-Yin Chu,2,*

1 School of Chemical Engineering, Sichuan University, Chengdu 610065, China

2 State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China

Keywords:Adsorption separation β-Cyclodextrin Bisphenol A Graphene oxide nanosheets Dopamine

A B S T R A C T A novel type of functional graphene oxide nanosheets(GNs)modified with β-cyclodextrins(β-CDs)have been developed by coating dopamine-functionalized cyclodextrin(DACD)molecules on GNs for removing Bisphenol A (BPA) molecules from water. The DACD molecules with both β-CD groups for achieving adsorption property and dopamine(DA)groups for achieving adhesion property are synthesized by grafting DA onto carboxymethyl-β-cyclodextrin(CmβCD).The proposed DACD molecules can be firmly coated on the surfaces of various inorganic and organic substrates.Due to the large specific surface area of GNs,DACD-coated GNs (DACD@GNs) are proposed for efficient adsorption separation of BPA molecules from water.Due to the host-gust complexation between the BPA molecules in water and β-CDs on DACD@GNs,the fabricated DACD@GNs exhibit excellent adsorption performances. The adsorption kinetics can be explained via the pseudo-second-order model effectively. The experimental adsorption capacity of DACD@GNs is 11.29 mg·g-1 for BPA. Furthermore, after the adsorption process, the DACD@GNs can be easily separated from aqueous solutions via vacuum filtration with porous membranes, and then regenerated by simply washing with ethanol. The proposed strategy in this study can be used for effectively functionalizing the surfaces of various substrates with functional β-CDs, which is highly promising in applications in the field of adsorption separations, especially water treatments.

1. Introduction

Organic pollutions in water,such as hormones[1,2],dyes[3-6]and pesticides[7,8],cause a severe threat to human health even at trace concentration [9-12]. The removal of such organic contaminants is one of the globally significant challenges at present [13].Affinity adsorption is a practical approach to the removal of trace toxic pollutants from water. β-cyclodextrin (β-CD) and its derivatives provide inclusion affinity to organic molecules by host-guest complexes,and show excellent adsorption properties to Bisphenol A (BPA, an endocrine disruptor), metolachlor (an insecticide), propranolol hydrochloride (a residual drug), 2,4-dichlorophenol(2,4-DCP, a carcinogen), and so on [1,14,15]. Therefore, the development of efficient β-CD-based adsorbent materials is very beneficial for the treatment of organic pollutants in water.

As for adsorption, highly efficient adsorption often requires large working surfaces of adsorbents. Therefore, materials with large specific surface area are the desired as substrates for the modification with β-CDs. Although some porous materials could provide high specific surface area, 2D materials such as graphene oxide (GO) nanosheets (GNs) with large specific surface area are better choice for the adsorber substrates for the modification with β-CDs. If the pores of porous materials are very small, like the framework-based materials with nano-sized pores, the pores might be clogged by the modified molecules. Furthermore, compared with porous substrates, 2D substrates provide a unique advantage for the easy transfer of molecules from solutions to the substrate surfaces, which is very beneficial for both the modification of substrates with β-CDs and the adsorption of organic pollutants from solutions. Take the typical 2D materials GNs for example, up to now many efforts have been made to modify β-CDs onto GNs by binding β-CD with GNsviacovalent bonds[16-19] and hydrogen bonds [20-22]. The oxygen-containing groups on GNs include hydroxy groups, epoxy groups, carboxyl groups, and so on. By esterifying the hydroxy of β-CD and the carboxyl of GNs, the β-CD can be covalently bound with GNs[16-19]. The strong hydrogen bonds exist between β-CDs and the oxidation groups of GO could be another strategy to prepare CD-functionalized graphene nanosheets [20-22]. Recently, Chekin et al [23] modified the reduced GO (rGO) electrochemical transducers with β-CD-modified dopamine (dopa-CD) through π-π interactions between the benzene rings of dopamine and the rGO. However, the existing modification methods of GNs with β-CDs are not universal,and a specific modification method is usually required for a special substrate. Therefore, development of a universal strategy for effectively functionalizing the surfaces of diverse substrates with β-CDs for efficient adsorption of organic pollutants is still challenging.

Here, we report on a novel strategy to fabricate β-CD-modified GNs by anchoring dopamine-functionalized β-CDs (DACD) onto GNs based on self-polymerization. First, the DACD molecules are synthesized from carboxymethyl-β-cyclodextrin (CmβCD) and dopamine hydrochloride (DA) (Fig. 1a). The DACD molecules are featured with excellent adhesion properties inherited from DA,which enable the proposed strategy in this study to be a universal one for effectively functionalizing the surfaces of diverse substrates with β-CDs. As mentioned above, the 2D GNs provide unique advantage for easy transfer of molecules from solutions to substrate surfaces,which is beneficial for both the modification with β-CDs and the adsorption of organic pollutants. Therefore,GNs are selected as substrate supporters of DACD molecules to fabricate DACD@GO nanosheets (DACD@GNs) (Fig. 1b). The Fourier transform infrared (FT-IR), X-ray photo-electron spectroscopy(XPS),atomic force microscopy(AFM)and thermogravimetric analysis(TGA)are used to characterize the physical and chemical properties of DACD@GNs. Bisphenol A (BPA) is selected as the model organic pollutant, and the adsorption performances of the DACD@GNs are demonstrated by removing BPA molecules from water (Fig. 1c). The results show that the DACD@GNs could efficiently adsorb the BPA molecules in water due to the hostgust complexation between the BPA molecules and CD molecules and could easily recovered by simple filtration on porous supports after using and regenerated by washing with ethanol. The proposed strategy in this study is highly promising in applications in the field of adsorption separations, especially water treatments.

2. Materials and Methods

2.1. Materials

Carboxymethyl-β-cyclodextrin (CmβCD) was purchased from Shandong Zhiyuan Biotechnology. Dopamine hydrochloride (DA)was purchased from Sigma-Aldrich. Bisphenol A (BPA) was obtained from Chengdu Kelong. Mixed cellulose esters (CN-CA,pore size: 0.22 μm) membranes were purchased from Millipore Express. All the chemicals were analytic reagents. Deionized (DI)water was purified by Milli-Q.

2.2. Preparation of DACD molecules

The DACD molecules were synthesized through the acylation reaction between carboxyl on CmβCD and amino on DA that catalyzed by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) (Fig. 1a) [24]. In the reaction between the carboxyl groups on CmβCD and the amino groups on DA,EDC could activate the carboxyl groups to form the intermediates, which were then combined with NHS to form semi-stable NHS esters,and finally reacted with primary amine groups to form amide groups.In this process,the NHS could greatly improve coupling efficiency. Briefly, 1 g of CmβCD was dissolved in 20 mL of phosphate-buffered saline (PBS) buffer solution (pH = 4.5). The mixed solution was deoxygenated using N2in ice bath for 30 min. Then, 1.12 g of DA was added into the solution, and continue deoxygenating in an ice bath for 20 min. Next, 3 g of EDC and 0.5 g of NHS were quickly added into the solution.The reaction took place at a temperature lower than 3℃ for 18 h with vigorous stirring, and then 180 mL ethanol was added to obtain the crude products (DACD and a small amount of residual CmβCD). The refined DACD products were obtained by centrifugation(2,000 r·min-1, 5 min) for 5 times with 90% (volume) ethanol as the washing solution. The final DACD products were obtained by freeze-drying.

Fig. 1. Schematic illustration of the synthesis of DACD (a), the coating of DACD molecules on GO nanosheets (b), and the separation of BPA molecules by DACD@GO nanosheets via affinity adsorption (c).

2.3. Adhesion property tests of DACD molecules

To demonstrate the adhesion property of DACD, silicon, glass,copper plates and polyurethane sponge were used as substrates to investigate the adhesion property of DACD molecules.The coating experiments of DA and DACD molecules on various substrates were performed in Tris buffer (pH = 8.5, 25℃) for 12 h.

2.4. Preparation of DACD@GNs

To demonstrate the adsorption property of DACD molecules,typical 2D materials GNs with large specific surface area were selected as substrate supporters for coating DACD molecules. The GNs were fabricated by a modified Hummers method [25,26],and were purified by dialysis for a week. The GNs were exfoliated by ultrasonic processing for 30 min. Then, the undispersed GNs were separated by centrifugation(3000 r·min-1,20 min).The exfoliated GNs were dispersed in DI water to form a suspension with GNs concentration of 5 mg·ml-1.Next,DACD@GNs were fabricated by the following steps. 0.2 g of DACD and 20 ml of GN suspension were added into 80 ml Tris buffer (pH = 8.5), and then homogenized by ultrasonic for 10 s. The coating process takes place at 25℃ for 12 h with vigorous stirring (Fig. 1b). Finally, the purified DACD@GNs were obtained by centrifugation (18000 r·min-1,40 min) for several times with DI water as washing solution. The dried DACD@GNs were obtained by lyophilization.

The coating amount of DACD on GNs were determined by the following equation:

where,wDACD(%) is the coating mass amount of DACD on GNs, andLDACD@GN(%),LGO(%)andLDACD(%)were respectively the mass reductions of DACD@GNs, GNs and DACD during a specific temperature range.

2.5. Adsorption tests of DACD@GNs

BPA adsorption experiments of DACD@GNs were carried out in 4 mL tubes with vortex mixers or magnetic stirrers. All the experiments were carried out at 25℃. Adsorption thermodynamic studies were carried out with addition of 1.5 mg of DACD@GNs in 3 ml BPA aqueous solutions with different concentrations. The mixture was dispersed adequately by magnetic stirring for 15 min, and then was filtrated by CN-CA membranes with the pore size of 0.22 μm. For the comparation of adsorption performances of DACD@GNs and GNs, a control group of experiments was carried out with GNs as the adsorbents. The concentrations of GNs and BPA were 0.5 mg·ml-1and 0.01 mmol·L-1respectively. After separating the nanosheets from the mixture,BPA concentration (ce) in supernatant was measured. The following equation was introduced to calculate the BPA removal efficiency (R):

where,c0is the initial BPA concentration (mmol·L-1), andceis the residual BPA concentration (mmol·L-1).

The Langmuir isotherm was introduced as the following equation:

where,qe(mg·g-1) is the mass of adsorbed BPA,qmax,cal(mg·g-1) is the theoretical maximum adsorption capacity,andKLis the adsorption equilibrium constant.

The Freundlich isotherm was introduced as the following equation:

where,nandKFare constants.

Adsorption kinetic studies were carried out with addition of 1.5 mg of DACD@GNs in 3 mL of BPA aqueous solution containing 0.01 mmol·L-1BPA. The mixture was promptly dispersed for various time lags(10 s, 0.5 min,1 min, 2 min,5 min and 15 min),and then was filtrated by CN-CA membranes with the pore size of 0.22 μm.A pseudo-second-order adsorption model was introduced to describe the uptake rate of BPA:

where,qt(mg·g-1)is the mass of adsorbed BPA at timet(min),andKobsis a second-order rate constant (g·mg-1·min-1).

2.6. Regeneration of DACD@GNs after adsorption

After finishing the adsorption of BPA,the DACD@GNs were separated from the aqueous solution by vacuum filtration with porous CN-CA membranes of pore size being 0.22 μm. Then, the DACD@GNs membrane cakes were taken to investigate the regeneration performance of the DACD@GNs. 40 ml BPA solution with BPA concentration of 0.01 mmol·L-1was penetrated through the membrane cake formed with 10 mg of DACD@GNs,and the filtrate at every fixed volume was collected for characterization of BPA concentration.To regenerate the DACD@GNs after each adsorption test, the membrane cake was washed by ethanol to remove the adsorbed BPA molecules from the DACD@GNs. The adsorption of BPA and the regeneration by washing with ethanol were operated for several cycles to show the regeneration performance of DACD@GNs.

2.7. Characterization of materials

The chemical structures of DA, CmβCD and DACD were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50). The degree of DA substitution was determined by1H NMR(Bruker AV II-400 MHz, D2O).

The morphology of DACD@GNs were detected by Atomic Force Microscope(AFM,Bruker MultiMode 8).The chemical composition of DACD@GNs was determined by FT-IR. To confirm the coating amount of DACD on GNs,thermogravimetric analyses of GO,DACD and DACD@GN were carried out by TGA (NETZSCH TG209F1, N2protection, 40 -1000℃, with heating rate of 10℃·min-1).

Droplet shape analyzer (KRUSS GmbH DSA25) was used to observe the change of surface wettability of substrates before and after coating with DACD. The morphology and stability of DACD-coated GNs were observed by AFM (Bruker MultiMode 8).The elemental analysis of DACD coating was performed by XPS(Kratos XSAM800).

BPA concentration in aquoues solution was measured by UV-vis spectrophotometer (λ = 276 nm, SHIMADZU UV-1800).

3. Results and Discussion

3.1. Chemical structures and adhesive properties of DACD molecules

To ascertain the chemical structures of DACD molecules, FT-IR spectra of all related substances are analyzed(Fig.2a).The characteristic peak of β-CD (1030 cm-1) is observed on the curve 1(CmβCD)and curve 3 (DACD).The typical peak of carbonyl groups at 1680 cm-1is found on the curve of DACD(curve 3),proving the successful bonding of amino groups of DA and carboxyl groups of CmβCD. The degree of DA substitution is given by the1H NMR spectrum (Fig. 2b). Hydrogen corresponding to DA (Ha1, Ha2) and CmβCD (Hb) can be found on DACD. Both the FT-IR and the1H NMR results confirmed that the expected reaction has occurred successfully.By calculating the ratio of Ha1,Ha2and Hb,the degree of DA substitution is calculated as 3.59.

XPS survey spectra of various coated substrates are shown in Fig. 3, and related results of the element contents are given in Table 1.In Fig.3a-c,there is no nitrogen on the surface of the original glass,silicon wafer and copper sheet.After coating with DACD or DA, nitrogen appears as a sign of coating formation. Because CmβCD is oxygen-rich,the oxygen content in DACD coating is significantly higher than that in DA coating (Table 1). The above results show that the DACD molecules have similar adhesion properties to DA molecules,and can be coated on the surfaces of various inorganic and organic substrates.

Table 1 XPS results showing element contents on the surfaces of various substrates.

The AFM images and water contact angle(CA) of the DACDcoated silicon wafer are shown in Fig. 4. The surface of the silicon wafer looks quite smooth (Fig. 4a). The roughness is only 0.7 nm with the water contact angle of 72.0°(Fig. 4b, c). After coated with DACD, the surface of the silicon wafer becomes rough (Fig. 4d). The roughness increases to 2.2 nm approximately (Fig. 4e). The water contact angle of the DACD-coated silicon wafer decreases to 36.0° (Fig. 4f), which confirms the change of the surface property. When the DACD-coated silicon wafer has been soaked in water with shaking for 6 days, the DACD coating still strongly adhere on the surface of silicon wafer(Fig. S1).

Fig.2. (a)FT-IR spectra of CmβCD(Curve 1),DA(Curve 2)and DACD(Curve 3).(b) 1H NMR(400 MHz,D2O)spectrum of DA(upper),DACD(middle)and CmβCD(bottom),in which the inset is the magnified part of DACD spectrum corresponding to Ha1 and Ha2 of DA.

3.2. Chemical structures of DACD@GNs

The GNs are stripped from the graphite by oxidation, so both sides of GNs have the same chemical structures statistically.Therefore, if there are excessive DACD molecules in the GN suspension,DACD molecules could be coated on both sides of GNs.In our study,excessive DACD molecules are added in GN suspension in the preparation of DACD@GNs. The AFM images of GNs and DACD@GNs are shown in Fig. 5. According to the results of AFM(Fig. 4 and Fig. 5), the thicknesses of DACD coating, GNs and DACD@GNs are about 2.2 nm, 0.8 nm and 5.4 nm, respectively.Therefore, the most reasonable explanation for the thickness variation from GNs to DACD@GNs should be that both sides of GNs are coated with DACD molecules on the DACD@GNs.

To confirm the chemical composition of DACD@GNs,FT-IR spectra and thermogravimetric analyses of raw materials and DACD@GN are compared and analyzed (Fig. 6). The characteristic peaks of β-CD (1030 cm-1) and carbonyl groups (1680 cm-1)appear on the curve of DACD (Curve 2) and DACD@GNs (Curve 3), which confirm that DACD molecules have been coated on the GNs. The amount of DACD coating on DACD@GNs is determined by TGA results (Fig. 6b). Section ①is contributed by evaporation,and the Section ③is the residual mass. As for Section ②, curves of both DACD and GNs have only one falling segment; however,there are two falling segments appearing on the curve of DACD@GNs which corresponds to the mass loss of DACD and GNs respectively. In Section ②, GNs, DACD and DACD@GNs have a mass loss of 41%, 56% and 44% respectively. According to the Eq. (1), the amount of DACD coating on DACD@GNs can be calculated as about 20 %(mass). According to the results of1H NMR(Fig. 2b), the degree of DA substitution can be calculated as 3.59,and thus the molecular weight of DACD can be roughly estimated as 2026.30 g·mol-1because the molecular weights of CmβCD and DA are 1541 g·mol-1and 153.18 g·mol-1respectively. Therefore,the complexing rate of DACD (or CD groups) on GNs can be estimated as 9.87 × 10-5mol·g-1.

Fig. 3. XPS survey spectra of DACD and DA coating on glass (a), silicon wafer (b), copper sheet (c) and polyurethane (d).

3.3. The adsorption performances of DACD@GNs

As shown in Fig. 7a, the DACD@GNs can adsorb 60% BPA from aqueous solution within 30 s. The adsorption may be caused by the host-gust complexation between BPA and β-CD as well as the π-π interactions between the benzene rings in BPA molecules and those in the dopamine and GNs of DACD@GNs. To further study the adsorption kinetics, the pseudo-second-order kinetic is introduced to match the experimental results. The coefficient of association (R2= 0.99989) demonstrates that the pseudo-second-order kinetics is applicable to the experiment data (Fig. 7b). The results show that the DACD coating on the DACD@GNs dominates the adsorption of BPA based on the host-gust complexation between BPA and β-CD. Comparing with the adsorption performances of DACD@GNs and GNs, GNs show much lower BPA adsorption efficiency in comparison to DACD@GNs (Fig. S2). The experiment result of the maximum adsorption capacity of DACD@GNs isqmax,exp= 11.29 mg·g-1(Fig. 7c). When the Langmuir and Freundlich isotherms are applied to fit the values of the adsorption, the Langmuir isotherm is more applicable than the Freundlich isotherm (Fig. 7d,

Fig.4. Characterization of DACD coating.(a)-(c)The 3D(a)and 2D(b)AFM images as well as the water contact angle(c)of the surface of silicon wafer without DACD coating.(d)-(f) The 3D (d) and 2D (e) AFM images as well as the water contact angle (f) of the surface of the same silicon wafer with DACD coating.Fig. S3). The result is keeping with another experimental result that the complexes of BPA/β-CD are of an association constant(KL) of 20,886 L·mol-1. According the fitted value, the calculated maximum adsorption capacity (qmax,cal) is about 13.68 mg·g-1,which is close to the experimental value.

Fig. 5. AFM images of GNs (a) and DACD@GNs (b).

Fig. 6. (a) FT-IR spectra of GNs (Curve 1), DACD (Curve 2) and DACD@GNs (Curve 3). (b) Thermogravimetric analyses of GNs, DACD and DACD@GNs under N2 atmosphere(60 ml·min-1) at a heating rate of 10℃·min-1.

Fig.7. Kinetics and thermodynamics of adsorption of BPA by DACD@GNs.(a)Time-dependent adsorption of BPA(0.01 mmol·L-1)by DACD@GNs(0.5 mg·mL-1).(b)Pseudosecond-order kinetics of BPA adsorption on the DACD@GNs. (c) BPA adsorption capacity by DACD@GNs. (d) Langmuir isotherm model of BPA adsorption by DACD@GNs.

3.4. Regeneration performances of DACD@GNs

After finishing the adsorption of BPA, the DACD@GNs can be easily separated from the aqueous solution by vacuum filtration with the porous CN-CA membranes. As shown in Fig. 8a-c, 5 mg,10 mg and 20 mg of DACD@GNs are filtrated into DACD@GNs membrane cakes coded as M1, M2 and M3 respectively. As the mass of the DACD@GNs increases, the thickness of the membrane cakes increases. The DACD@GNs membrane cakes are formed by stacking DACD@GNs layers, so they can be used as membranes.The water fluxes of the DACD@GNs membranes decrease with increasing the thickness, and are 610 L·m-2·h-1·bar-1,384 L·m-2·h-1·bar-1and 147 L·m-2·h-1·bar-1(1 bar = 0.1 MPa)for M1, M2 and M3, respectively. Based on the water flux data, it can be inferred that DACD@GNs can be separated from the aqueous solution smoothly. Besides, there is a positive linear relationship between water flux and operating pressure (Fig. 8d), which conforms to the Poiseuille’s law. Because the operating pressure is not high, the incompressible fluid moves in laminar flow in the narrow channels of membrane cakes. As a result, a positive linear relationship between water flux and operating pressure exists as the structure of membrane cake is fixed. The result means that a faster separation operation can be achieved by increasing the pressure.

After separated from the aqueous solution,the used DACD@GNs can be regenerated easily by washing with ethanol, and the BPA removal efficiency can be recovered back to almost 100%even after several repeated running cycles (Fig. S4). The results demonstrate that the proposed DACD@GNs are featured with excellent regeneration performances.

Fig.8. (a)-(c)SEM images of cross-sectional views of M1(a),M2(b)and M3(c)DACD@GNs membrane cakes.The scale bar is 5 μm.(d)Relationship between water flux and operating pressure of M1 (a), M2 (b) and M3 (c) DACD@GNs membrane cakes.

4. Conclusions

In summary, functional DACD@GNs with efficient adsorption performances for removing BPA molecules from water have been developed by coating DACD molecules on GO nanosheets in Tris buffer at pH = 8.5. The DACD molecules with both CD groups and adhesive property have been synthesized by grafting dopamine onto carboxymethyl-β-cyclodextrin (CmβCD). The proposed DACD molecules can be coated on various inorganic and organic surfaces to achieve adsorption of organic pollutants in waterviathe hostguest inclusion complexations. Due to the large specific surface area of GNs, the proposed DACD@GNs are featured with efficient adsorption performance for removing BPA molecules from water.The experimental results show that the prepared DACD@GNs have an adsorption capacity of 11.29 mg g-1for BPA. Furthermore, the DACD@GNs can be easily separated from aqueous solutionsviavacuum filtration with porous membranes and have excellent regeneration properties. Although BSA is selected as the only subject to conduct the adsorption test in this work, the DACD@GNs adsorber can work for other similar toxic organic pollutants, such as hormones, dyes and pesticides, due to the host-guest inclusion complexations between β-CDs and organic pollutants [1,14-17].Therefore, the proposed β-CD modification method, which can be used for functionalizing various substrates with β-CD groups, is highly promising in applications in the field of adsorption separations, especially water treatments.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21490582).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.02.018.